How important is the cable between the SUT and phono pre?

Really, really important. Ultra low capacitance, and low run length < 1.0m are key. You need to try to keep it under 50 pF. Don’t use a "normal" thick audiophile interconnect.

Before I knew how important low capacitance was, I did some cable rolling with high quality "normal" interconnects. All the 1m cables had their sonic signature GREATLY magnified, to the point of obvious coloration. The AQ Fire 1m became overly detailed and aggressive. The AQ Sky 1m sounded overly warm, thick and and bloomy. The 0.5m AQ Niagara, normally inferior to the higher-end Fire and Sky, did better and was relatively uncolored - because of the halved length and significantly lower capacitance. The the dedicated AQ WEL LP phono cable (optimized for lower capacitance), 0.5m, blew them all away. The Bob’s Devices 0.7m interconnects are also really good (but the WEL LP is still better) - I have both their copper and silver cables. They’re optimized for SUT use, obviously.

This was all tested on the same Koetsu cartridge and Bob’s Devices Sky SUT.

MC are said to not be sensitive to capacitance, but the problem with a SUT is that it reflects the capacitance of its output cable times the SQUARE of its turns ratio. That’s enough capacitance that it WILL start to affect the MC cartridge.

Many Cardas interconnects have very, very low capacitance - so if you've got those in your collection, give 'em a try. I believe Bob's uses Cardas ingredients to make his cables.

@billwojo

I am probably not the kind of expert you need here. But look at this page which I found very interesting:

http://www.hagtech.com/loading.html

The first section deals specifically with loading MM cartridges with capacitance. However, the formula Fr = 1 / (2*PI*SQRT(L * C)) works equally for MC and MM cartridges. This formula defines the cart’s resonant frequency based on its inherent inductance and the downstream (cable) capacitance. The resonance is often accompanied by a sharp peak and followed by rolloff. Generally you want this frequency as high as possible, well out of the audible band (> 20kHz). This generally requires low capacitance OR low inductance (as per the formula). MC cartridges get the low inductance (that’s the L in the formula) for "free". Normally this is enough that they need not concern themselves with phono cable capacitance. HOWEVER with a SUT reflecting the output cable’s capacitance times the square of its turns ratio (look this up elsewhere), capacitance could suddenly become large enough to drop the resonant frequency back near or into audible range and become a concern. For example: say you (unwisely) choose a fat audiophile interconnect run with 200 pF capacitance. And your SUT is a 30x. 30 * 30 * 200pF = 180,000 pF. Wow. Say your MC has an inductance of 1mH. Using the calculator in that Hagerman page I linked, the resonance is now at 3,800 Hz. You’re gonna have a bad time! Choose a cable with 40pF and now it’s at 26,000 Hz. Much better!

Agree with @mijostyn keeping your run out of the SUT super short (0.5m or less) is probably what you should focus on in most cases. 

@jpjones3318 No I don't have a cite for that part. I've seen it mentioned a couple times on forums, but forget where / who. I took that, combined with the info on the Hagerman page (which was fascinating to me), and my own SUT + cable listening tests (more capacitance, longer runs after SUT == BAD) and ran with it haha.